Permeable hollow-fiber membrane processes and systems are being employed or considered for a wide variety of fluid (gas and liquid) separations. In such operations, a feed stream is brought into contact with the surface of the membrane; a pressure differential is maintained across the membrane; the more readily permeable component of the feed stream passes through the membrane and is withdrawn as the permeate stream; and the less readily permeable component does not pass through the membrane and is withdrawn as a non-permeate, or retentate, stream.
The membrane material and form employed may be any suitable material capable of effecting the desired separation. For example, cellulose derivatives, polyamides, polyimides, polysulfones, and polystyrenes have found utility. Also for example, hollow-fiber membranes may be composite, asymmetric, or dense film.
The membrane is generally supported and sealed in a housing such as a shell or vessel to form a permeator. The housing contains the fluids, protects the membrane, and channels and separates the flow of the permeate and retentate. Particularly in the case of membranes comprising hollow fibers, the art has taught that more than one bundle of hollow fibers can be included in a single housing. Such an arrangement reduces weight and cost of the system.
U.S. Pat. No. 3,884,814 describes a hollow fiber permeator having two "U" shaped bundles on a cruciform core in a single housing. A lid equipped with an inlet and an outlet for each of the "U" shaped bundles permits feeding the bundles separately. The reference suggests ultrafiltration of different solutions in each bundle.
U.S. Pat. No. 3,953,334 teaches a plurality of separate bundles of hollow cords separately supported and sealed in side plates connected to a central conduit and support within a single shell. Flange members provide common fluid communication between the ends of the cord bundles and the outside of the shell.
A permeator is generally designed to provide a given product quality (separation) at a single flow rate (flux). U.S. Pat. No. 4,806,132 claims a method of achieving turndown control in a permeator system at reduced flow demand. The background of that patent suggests without any detail that turndown can be achieved in other systems by shutting down a portion of the membrane area in the system under reduced demand conditions.
U.S. Pat. No. 4,397,661 employs a plurality of permeators manifolded in a manner that permits shutting down a portion of the total area of membrane in the system under reduced demand conditions by valving one or more permeators out of the system.
U.S. Pat. No. 4,537,606 teaches a method of varying oxygen flow and concentration for combustion by varying total membrane area by valving in auxiliary membrane cells manifolded with the primary cell.
One use for permeators described in U.S. Pat. No. 4,556,180, which is incorporated herein by reference, is inerting of fuel tanks on airplanes to eliminate explosive gas mixtures that are a hazard in case of lightning strikes, crash damage or military damage. Particularly in situations where feed air is at a low pressure such as on helicopters, an alternate system employing one or more air separation modules is taught.
U.S. Pat. No. 4,556,180 does not provide a means for varying flow from the permeator based on demand as is particularly needed in airplane operation. For example, during level flight, a relatively low rate of nitrogen-enriched-air (NEA) flow is required to take the place of fuel being used. During a dive from high altitude, however, a higher rate of NEA flow is required to keep the internal pressure in the fuel tanks equal to the external pressure. If the fuel tanks are nearly empty during a dive, an even higher rate of NEA flow may be required.
Manifolding permeators such as in U.S. Pat. No. 4,397,661 would enable varying flow while maintaining NEA with the percent oxygen in the gas at less than 9 percent. But a hollow-fiber permeator system needed to accommodate the various flow needs noted above would require at least two separate permeators which are heavy and bulky and therefore are a problem on an aircraft with weight and space limitations.
Simply operating a single conventional permeator at the several different flow rates needed is not desirable for the following reasons: when operating a high flow capacity permeator at lower flow rates, the velocity of gas through the hollow fibers is so low that the recovery (ratio of NEA flow to feed flow) of the permeator drops to an inefficient level; and when operating a low flow capacity permeator at higher flow rates, the purity of the NEA drops to an undesirably low value (percent oxygen greater than 9%).